Solid state lights (“SSLs”) use solid state emitters (“SSEs”) as sources of illumination. Generally, SSLs are more energy efficient, generate less heat, provide greater resistance to shock and vibration, and have longer life spans than conventional lighting devices that use filaments, plasma, or gas as sources of illumination (e.g., florescent or incandescent lights).
A conventional type of SSL is a “white light” SSE. White light requires a mixture of wavelengths to be perceived by human eyes. However, SSEs typically only emit light at one particular wavelength (e.g., blue light), so SSEs must be modified to emulate white light. One conventional technique for modulating the light from SSEs includes depositing a converter material (e.g., phosphor) on the SSE. For example, FIG. 1A shows a conventional SSL 10 that includes a support 2, an SSE 4 attached to the support 2, and a converter material 6 on the SSE 4. As shown in FIG. 1B, the SSE 4 can include one or more layers of semiconductor material such as an N-type gallium nitride (“GaN”) material 14, a light-emitting indium gallium nitride (“InGaN”) material 16 and/or InGaN/GaN multiple quantum wells, and a P-type GaN material 18 on one another in series supported by a substrate 12. The SSE 4 can be a lateral-type device that includes a first contact 20 on the P-type GaN material 18 and a second contact 22 on the N-type GaN material 14 spaced laterally apart from the first contact 20. Referring to both FIGS. 1A and 1B, the SSE 4 emits blue light that stimulates the converter material 6 to emit light at a desired frequency (e.g., yellow light). The combination of the emissions from the SSE 4 and the converter material 6 appears white to human eyes if the wavelengths of the emissions are matched appropriately.
SSLs including multiple SSEs arranged in an array are becoming popular because they produce a greater luminous flux output with a smaller footprint, and also reduce the cost of packaging. FIG. 2A is a schematic cross-sectional view of a conventional multi-SSE device 50 having a support 52, a plurality of SSEs 4 attached to the support 52, and a converter material 56 over the support 52 and the SSEs 4. The multi-SSE device 50 also has a single lens 58 over the SSEs 4. The SSEs 4 are connected to a common anode and cathode such that all of the SSEs 4 operate together. FIG. 2B is a schematic top plan view of a common pattern for the SSEs 4 in conventional multi-SSE devices. Conventional multi-SSE devices typically have a two-dimensional grid of identical SSEs in which adjacent SSEs are spaced apart from each other by a constant distance throughout the entire array. In other conventional multi-SSE devices, arrays include assorted SSEs randomly arranged and spaced apart from each other throughout the array.
To select SSEs for a particular array, SSEs can be categorized by performance specifications (e.g., luminous flux, peak emission wavelength, forward voltage, and/or other performance criteria) in a process known to those skilled in the art as “binning.” During the binning process, individual SSEs are given a performance specification rating for a desired property at a set temperature, and placed in a category with SSEs having the same performance specification rating. For example, SSEs can be rated based on their luminous flux at room temperature, 25° C., and SSEs having a luminous flux rating in a given range can be placed in the same category (e.g., Bin 1). The SSEs in conventional arrays are generally selected from a single bin, such that all of the SSEs in an array have the same performance specification rating.
One drawback of such conventional multi-SSE devices is that the temperature across the device can vary (e.g., 10° C. variance) and cause undesirable outputs. Lines 2X and 2Y in FIG. 2B illustrate a temperature gradient across the conventional multi-SSE device 50 in which the temperatures are higher toward the center of the device 50 than the periphery. The temperature affects the output and other performance criteria of SSEs (e.g., thermal derating). For example, SSEs having the same performance specification rating at 25° C. may decrease luminous flux output by 15% at 85° C. and 20% at 125° C. The SSEs toward the center of the device accordingly have a lower output or otherwise perform differently than the SSEs around the periphery. Additionally, heat causes some components of SSLs to deteriorate and fail over time. For example, phosphor coating deteriorates faster at higher temperatures, thus changing the color of the light emitted by the SSL, and the junctions in the light producing materials deteriorate from exposure to heat, thereby reducing the light output at a given current (i.e., reduces the efficiency). Therefore, the SSEs at the center of a multi-SSE device operate differently and have shorter life spans than the SSEs around the periphery of the device. Disadvantageously, this creates a non-uniform output across the multi-SSE device.